Not sure this is exactly what you are looking for but I have been doing some research for similar reasons.
Payment strategies: Unscramble the alphabet soup of medicare payment systems
>The Medicare Catastrophic Coverage Act of 1988 requires health care professionals to include an appropriate ICD-9-CM code for each procedure, service, or supply billed to Medicare. To comply with the regulations, health care professionals must convert the reason the patient needs procedures, services, or supplies from written statements that include specific diagnosis, signs, symptoms, and/or complaints into ICD-9-CM diagnosis codes. The diagnosis code used should be the one at the highest level of specificity. If a 5th-digit subclassification is provided, the provider must use the 5th-digit code.
My research is ongoing so I'll let you know what else I find.
I think I found something more specific but can't remember where!
Thanks & in the same boat as you,